The
Colorado Plateau is a physiographic "province," a region geologically
and topographically distinct from other parts of the West. Originally
named the "Colorado Plateaus" by explorer John Wesley Powell,
the "Plateau" is in fact a huge basin ringed by highlands and
filled with plateaus. Sprawling across southeastern Utah, northern
Arizona, northwestern New Mexico, and western Colorado, the Colorado Plateau
province covers a land area of 130,000 square miles. Of America's 50 states,
only Alaska, Texas, California, and Montana are larger.

Asked to explain what makes the Colorado Plateau unique, geographers
grow cryptic, enigmatic, even mystical. Perhaps that is inevitable, for
nothing is more typical of the "Plateau" than enigma itself.
Geologically, it is perhaps best defined by what did not happen
to it. While the Rocky Mountains to the east and the basin and range country
to the west were being thrust, stretched, and fractured into existence,
the Colorado Plateau earned a name for itself by the simple device of
remaining structurally intact.

"The Colorado Plateau is extremely ancient," says author F.A.
Barnes, an expert on the region's geology. "As a distinct mass of
continental crust, it is at least 500 million years old -- probably a
lot older." Such longevity is especially impressive when one considers
the globetrotting adventures of the North American continent from the
perspective of continental drift theory. Over a period of 300 to 400 million
years, while the land mass that would become the North American continent
inched northward from the South Pole, gradually disengaging itself from
Africa, Asia, and South America, the Colorado Plateau region drifted along
comfortably on its western edge. Now shoreline, now inundated by rising
seas, the entire region accumulated huge quantities of sediment, gradually
sinking under its own weight until heat and pressure hardened the deposits
into a mantle of sedimentary rock several miles thick. Even when the entire
western United States began to rise some 10 million years ago, eventually
climbing to elevations as much as three miles above sea level, the Colorado
Plateau region remained stable  perhaps "floating" on
a cushion of molten rock.

Though volcanic eruptions ring its perimeter, few have penetrated the
interior of the Colorado Plateau. Blocked by massive layers of sedimentary
rock, rising magma could do no more than bulge its thick roof into domes
-- the "laccolithic" Henry, La Sal, and Abajo mountain ranges
-- before cooling and hardening in place. The tremendous tectonic forces
which formed the Sierra Nevada and Rocky Mountains had far less effect
on the Colorado Plateau. Shielded or cushioned by something deep in the
earth, the Plateau mirrored those forces but dimly -- as broad, dome-shaped
uplifts, shallow basins, and long folds or "reefs." [Photos]